In an interferometer, such as a Michelson-type interferometer, a light beam coming from a light source is split into two beams by a beamsplitter. The first beam travels to a fixed mirror and the second beam to a movable mirror. After being reflected, the beams recombine and interfere at the beamsplitter. The optical path difference, i.e. the difference in the optical path lengths travelled by the first and the second beam, results in a phase difference between the beams. By moving the movable mirror in the direction of the second light beam, the phase difference between the light beams changes. This change in phase difference modulates the intensity of the interference beam which is detected by a detector.
In order to achieve high accuracy, the plane of the reflective surface of the movable mirror must be aligned perpendicular to the direction of the light beam directed to it. The perpendicular alignment must also be maintained while the movable mirror is moving. In other words, the movable mirror should be moved in such a manner that the mirror does not tilt.
Various supporting structures for a movable mirror have been described. A well-known structure, an example of which is disclosed in the document US 2002/0149777 A1, comprises a movable frame to which the mirror and a voice coil are attached. The movable frame is connected to a fixed frame by two equal length flexible members. The upper ends of the flexible members are connected to the movable frame, and the lower ends of the flexible members to the fixed frame in such a manner that the spacing between the upper fixing points equals the spacing between the lower fixing points. A permanent magnet is arranged in connection with the voice coil. By adjusting the current fed into the voice coil the distance between the voice coil and the magnet changes, whereupon the movable frame moves.
Movement of the movable frame from its rest position forces the flexible members to bend from a straight line into an S-curve. A disadvantage is that the flexible members do not bend equally whereupon the position of the movable frame relative to the horizontal plane alters. This results in the tilting of the movable mirror which decreases the accuracy of the interferometer.